How the Influence of Different Salts on Interfacial Properties of Surfactant-Oil-Water Systems at Optimum Formulation Matches the Hofmeister Series Ranking

In this work, we present the effects of salts on sodium dodecyl benzene sulfonate micellization and on the interfacial performance of a sodium dodecyl benzene sulfonate-heptane-brine system at optimum formulation, i.e., hydrophilic-lipophilic deviation (HLD) = 0. In order to do that, interfacial tension and dilational interfacial rheology properties of surfactant-heptane-water systems at optimum formulation are measured using an interfacial spinning drop tensiometer with an oscillating velocity, which can accurately measure interfacial rheology properties at both low and ultralow interfacial tensions. The brines used contain one of the following salts: MgCl2, CaCl2, NaCl, NH4Cl, NaNO3, CH3COONa, or Na2SO4. We performed a one-dimensional salinity scan with each of these salts to achieve an optimum formulation. In relation to the Hofmeister series, we found that, at optimum formulation, systems with chaotropic ions (NH4+, NO3-) present interfaces with ultralow interfacial tensions, very low dilational modulus, and a low phase angle, whereas kosmotropic ions (Mg2+, Ca2+, SO4-2) generate high interfacial tension and high rigidity monolayers. Intermediate ions in the Hofmeister series (Na+, CH3COO-, Cl-) present interfaces with intermediate properties. Furthermore, according to the Hofmeister series, interfaces can be respectively ordered from higher to lower rigidity for surfactant counterions Mg2+ > Ca2+ > Na+ > NH4+ and coions SO42- > CH3COO- > Cl- > NO3-, which correspond to a salting-out (highest rigidity) and salting-in (lowest rigidity) effect. We observed that counterions have a more significant effect on surfactant-oil-water system properties than those that act as coions.